The present invention relates to video measurements, and more particularly to a wander gamut display for indicating low frequency jitter in a video distribution system.
The measurement of low frequency jitter or wander is more important today in data transmission than it has ever been. The telephone companies have long recognized the importance of long term stability and low frequency timing perturbations of the “significant instants” for error free data transfer over SONET and SDH networks. A wander measurement addresses the low frequency or long period timing errors, often termed wander or very low-frequency jitter. In SONET and SDH systems the accuracy and stability of clocks is very important for proper data recovery. To provide a concise measure of synchronization quality, several wander parameters have been defined by the ETSI (ETSI EN 300 462 “Transmission and Multiplexing™; Generic Requirements for Synchronization Networks”. Part 1-1: “Definitions and Terminology for Synchronization Networks”) and ITU-T (ITU_T Rec G.810 “Definitions and Terminology for Synchronization Networks”) for SONET/SDH TelCom transmission and are used to specify performance limits. Some examples are Time Interval Error (wander in ns—TIE), Maximum Time Interval Error (related to a variable window of TIE—MTIE), and Time DEViation (TDEV). TIE is defined as the time-error of the significant instances between a signal being measured and a (wander-free) reference clock. MTIE is a measure of wander that characterizes wander as a function of duration or variable windowed segment of the TIE. TDEV is a statistical measure derived from the well-known Modified Allan Variance for characterizing frequency standards. MTIE and TDEV are often specified in terms of masks outlining the maximum limits allowed. Unfortunately wander is a problem that is not limited to SONET and SDH systems, but is also important in clock recovery and jitter measurements of data transmission in general. There are many instruments that measure the timing metrics associated with jitter, but it is often not clear how practical jitter measurements should process the wander component and how the timing metrics can be quantified as something more meaningful and practical for wander as it relates to subsequent clock recovery, particularly with regard to video data transmission.
Today's video distribution has many variable rate processing elements, such as MPEG processing, but must eventually be isochronal for quality image and audio presentation. In isochronous and synchronous systems the low frequency jitter and wander components may be very large in terms of phase-error or time-error (typically measured in seconds) without causing data loss since the clock is recovered with a phase locked loop (PLL) and the phase variations are tracked. Loss of data only occurs when the higher order time derivatives of the phase (frequency offset and frequency drift rate) exceed the tracking limits or specifications of the clock timing recovery or PLL. For example the average frequency offset has to be within the lock range of the PLL for lock to occur so this is an important performance limit to be measured. Even if the frequency offset is within limits, the frequency drift rate or slew has to be limited since virtually all clock recovery systems (PLLs) track with a static phase error that increases with frequency slew rate. If this static phase error is too much, intermittent data errors occur. In addition, jitter measurement systems may be affected by a wander component causing mysterious discrepancies in jitter measurements among different manufacturers of the jitter measurement equipment. Unfortunately there are few measurement standards and virtually no equipment to detect and quantify the wander and properly reject wander from jitter measurements. The methods of MTIE and TDEV are not well suited for video specifications that prefer to specify video wander in terms of frequency offset and frequency drift rate, and have not been adopted by the video industry.
Even in the midst of modern digital signal processing methods, video is often eventually converted to a composite analog form, such as the NTSC or PAL standard, where a subcarrier is used for the color information to achieve compatibility with monochrome televisions. NTSC and PAL standards specify a maximum frequency error and frequency drift rate for the color subcarrier and, since virtually all composite signals today are derived from a single recovered clock, this maximum frequency error and frequency drift rate is then imposed on the recovered system clock of any digital transmission of video data. Therefore it is important to quantify timing wander in terms of both frequency offset and frequency drift rate on serial digital video data interfaces as well as baseband analog video distribution.
Tektronix, Inc., an Oregon corporation, has provided needed wander measurements on baseband video with the VM700 Video Measurement Set, on serial-digital video with the VM700-1S Video Measurement Set and on MPEG video with the MTS-300 MPEG Test Set and the MTM-400 MPEG Transport Stream Monitor. These products measure and quantify both jitter and wander as separate timing errors and properly separate the wander components from the jitter measurement and vice versa. Unfortunately for the wander measurement these products produce sliding window time graphs lasting only thirty seconds or less. These are hard to use since long periods—minutes or hours—are required to verify compliance and the window period is typically not long enough to graph the long period wander, requiring the operator to watch the trace rise and fall in the sliding window to assess the nature and extent of the wander. By using alarm triggered time stamps it is possible to log wander limit violations, but the nature or wave shape of the wander is not captured with so short a time window.
What is needed is a simple to interpret display for compliance validation that is orthogonal to time such that a memory of all wander variations is retained in order to assess limit violations and graphically represent the nature or signature of the wander.